Sensorineural hearing loss results from damage to a variety of structures within the cochlea and auditory nerve. Presently, genetic and tissue transplantation strategies are being explored as therapeutic strategies for curing hearing loss. Unfortunately there are no available diagnostic techniques that isolate and distinguish the anatomic site(s) of damage within the cochlea or auditory nerve. A test battery that identifies the locus of lesion(s) will allow physicians to target the appropriate anatomic sites for medical intervention. The long-term goal of this research is to develop these diagnostic tests. In this proposal, the aim of the R21 phase is to determine if the cochlear microphonic (CM), in response to a low-frequency tone embedded in high-pass noise, can be used to quantify the location of missing outer hair cells along the cochlear partition. A cumulative distribution function (CDF) of outer hair cells is obtained by plotting the amplitude of the masked CM relative to the unmasked amplitude for specific masker frequencies. We hypothesize that the location of a plateau in the CDF (or some other variable in the CDF or corresponding probability density function) will relate to the location of missing outer hair cells. This is tested by correlating the location of missing outer hair cells estimated by the CDF with anatomic measurements in animals with hair cell lesions. In clinic populations, outer hair cells may be present but functioning abnormally. In the R33 phase we develop indices from hair cell transducer functions to determine the health of surviving outer hair cells. We hypothesize that 1) transducer function indices can be obtained along the length of the cochlea and 2) the indices can quantify outer hair cell health despite damage to other cochlear structures. The first goal is tested by correlating the location of the plateau of the CDF of an index with the location of maximum hearing loss in animals with temporary hearing loss at various frequencies. The second hypothesis is examined by comparing indices among groups of animals with lesions to outer hair cells and other structures. The studies in this proposal are likely to be translated to human patients. Presently, the CM is recorded in Audiologic clinics in the United States and Europe. Future studies optimizing stimuli and recording location are envisioned as the next step for developing diagnostic tests that compliment the new medical treatment strategies that are being explored to cure sensorineural hearing loss.